Chromium plating



4 Nov. 4, 1930. w. M. PHILLIPS CHROMIUM PLATING Filed June 30', 1926 Patented Nov. 4, 1930 UNITED STATES PATENT ,OFFICE- WILLIAM M. rnI 'LIPs, OF BIRMINGHAM, MICHIGAN, ASSIGNOR, BY MEs vE AssIeIi- MENI's, T0 GENERAL MOTORS RESEARCH CORPORATION, 0E DETROIT, MICHIGAN, A

CORPORATION OF DELAWARE A CHROMIUM PEATING Application filed June 30,

The invention' relates to the electro deposition of chromium and is more particularly concerned with the nature of the anode to be employed inthe electro-plating bath. Various materials have hitherto been suggested for this purpose including platinum, lead, lead peroxide, graphite, metallic chromium and ferro-chrome.

I have found, however, that metallic iron, for example in the form of ordinary low carbon steel, may be used and that it affords sub-, stantial advantages over any other anode material of which I am aware.

The principal object of the invention thereforeis the production of a novel chromium plating process involving the use of'an anode of ferrous metal. A further object is the provision of a chromium plating apparatus in which an anode of ferrous metal is one'of'the elements.

Other objects and advantages will appear from the following description taken in connection with the accompanying drawing which shows, in a vertical sectional view, a chromium plating apparatus embodying certain features of the invention.

' Referring to the drawing: 10 indicates a tank serving as a receptacle for a bath 11, the latter comprising a solution suitable for electrolytic deposition of chromium. Satisfactory results may be obtained by the use of baths having chromic acid 01:0. as a major constituent of the materials in solution, togetherwith another compound of chromium in which the chromium possesses a valence lower than that in the chromic acid, for example, an aqueous solution of. chromic acid with a relatively small amount of chromium sul hate, which is commonly referred to as Sargents solution. a k

The chromic acid generally obtainable from commercial'sourcesmften contains a considerable proportion'of impurities particularly sulphuric acid and sulphates. These im urities are the cause ofmarked'irregularitle's in the operation of the bath for depositing chromium and I have found it essential to emplov chromic acid of a relatively high degree of purity, for example, not iless than 97% pure.

192 Seria1No. 119,663.

The material of the inner surface of the tank or receptaclelO should, of course, be such as will not be attacked by the ingredicuts; of the plating bath, and I have found that iron may be advantageously used for this purpose. Accordingly I have shown the tank 10, as an iron receptacle which is preferably formed of ordinary sheet steel. Since, in accordance with my invention, the anode is also to be of ferrous metal, I may employ the walls of the container itself as the anode. The container is therefore connected, as by conductor 12, to a suitable sonrce of electric current, as the generator 13. The opposite pole of the generator may be. connected by conductor 14 to a bus bar 15 from which the article, to be plated, indicated at 16, may be suspended in the plating bath. The bus bar may rest upon the tank, beinginsulated therefrom by non-conducting blocks 17. r p In some cases it may be desirable, owing to the relatively poor throwing power of the chromium plating bath, to form the anode to conform more-or less closely to the contour of the surface to be'plated. The ferrous anode lends its'elf very readily to such use since it can be readily stamped or otherwise formed to the desired shape. n

I have shown, for example, an arrangement suitable for depositing chromium upon the interior of a concave object such as a lamp reflector. The reflector is indicated at 18 and the anode 19, which may constitute a wall of the tank, is shown as of sheeet metal conforming approximately to the contour of the surface to be plated.

It will be understood that the'use of iron or steel tank or anodes or'both is dependent upon the maintenance of the ingredients of the bath'or the'plating conditions therein such that there is no material corrosion of the metal of the tank or anodes. With baths of the Sargents solution type, for example,

bath provided they do not introduce a conditionpr a substance tending to increase unduly the corrosion of the anode material.

For example, the operation of the bath above referred to has been found to be improved by a small amount of boric acid or borates. The presence of such ingredients does not result in any appreciable deterioration of the anodes.

Long continued use of a sheet steel tank as an anode with such baths has shown that it undergoes substantially no change by reason of the action of the ingredients of the bath or the plating operation.

The cheapness of steel and its workability, as well as its peculiar resistance to corrosion in contact with the plating bath, render it a particularly convenient and inexpensive material for use both for the tank and the anode.

Furthermore, the use of such anodes tends to preserve the predetermined relative electrode areas due to the permanence of the surface characteristics and conducting capacity of'the metal.

I claim: 1. The process of depositing substantially pure metallic chromium upon conducting surfaces comprising electrolyzing a bath containing chromic acid as the principal constituent of the materials in solution and substantially free of any other constituent tending to attack iron during the deposition of the chromium, a metallic anode being employed the principal constituent of which is iron.

2.- The process as set forth in claim 1, the anode being of low carbon steel.

3. The process of depositing substantially pure metallic chromium upon conducting surfaces comprising electrolyzing a bath containing chromic acid as the principal constituent of the material in solution and having a sulphate content of not more than approximately 12% grams per liter of solution, a metallic anode being employed, the principal constituent of which is iron.

In testimony whereof I afiix my signature.

WILLIAM M. PHILLIPS. 

